1. Field of the Art
The present invention relates to a control method and control apparatus for a displacement-type fluid machine to control the number of revolutions of a drive motor by means of an inverter or the like in operating a displacement-type fluid machine such as a Roots-type blower or a vane pump.
2. Prior Art
A displacement-type fluid machine for handling fluid such as a displacement-type pump is, for example, employed for lowering or decreasing pressure on an intake side of a displacement-type pump, increasing the pressure on a discharge side of the pump, or transporting liquid across the pump. A displacement-type pump is normally used together with a sealable container such as a tank, and processing values such as pressure and liquid level within the tank, etc., are detected and controlled so as to be within a predetermined range, by actuating or stopping the displacement pump. When an inverter or the like is employed to adjust the number of revolutions of the motor, the frequency is gradually increased or decreased to avoid abrupt acceleration or deceleration upon driving or stopping, or the number of revolutions is selected according to fluctuations in process values.
In a so-called ON/OFF control method, a displacement-type pump is actuated when an allowable limit of a process value is detected and the pump is stopped when a pre-determined process value is detected. In this control method, however, the pump may be actuated too frequently depending on the operating conditions, resulting in damage to the motor and related equipment and a decrease in the working life of the equipment. In order to restrain the actuation frequency to within allowable times, a sealable container such as a tank must have sufficient capacity, which leads to increased facility costs. Further, since abrupt changes in process values are unavoidable in the ON/OFF control method, great fluctuations in the pressure or liquid level on the intake side or discharge side of the pump are caused, preventing stable operation of the system. Moreover, since the aforementioned control method greatly relies on detectors for detecting a pressure and liquid level, a proper operation of the apparatus is often prevented by the malfunctioning of these detectors.
The present invention has been made in the light of the aforementioned problems, and the object thereof is to provide a method and an apparatus for controlling a displacement-type fluid machine which enables the process values to be kept within an allowable limit without effecting repeated actuation and stopping of the pump.